1. Field of the Invention
This invention relates to an improved method of, and device for, on-line and out-of-service inspection and evaluation of internal, loose polymeric liner in piping and equipment. In particular, a nondestructive method for determination of the presence or absence of defects is accomplished, before actual leakage occurs, by an in-situ measurement of vapor permeability through the liner itself.
2. Description of the Prior Art
Plastic liners in process equipment are usually intended for corrosion protection of the underlying structural shell. Liners may become defective due to manufacturing and/or in-service related considerations. Liner defects such as environmentally assisted cracking, blistering, erosion, manufacturing defects and other defects have been observed in lined piping and equipment. While these defects may not present an immediate leakage symptom, they may limit the remaining useful life of the asset.
Conventional nondestructive testing (NDT) techniques such as visual inspection of the exterior shell, radiography, eddy current, ultrasonic, microwave and other methods cannot reliably assess the character of defects in a loose, plastic liner contained inside a structural shell before actual leakage occurs. Several methods and devices have been disclosed for detection of leaks in lined piping and/or double-walled piping. However, no method has been disclosed to nondestructively detect defective liners which pose an increased risk of loss of integrity.
U.S. Pat. No. 5,375,457 to Trapp (Dept. of Energy) discloses a method and device for detecting the location of leaks along a wall or piping system, preferably in double-walled piping. The apparatus comprises a sampling probe, a rigid cord such as a length of tube attached to the probe on one end and extending out of the piping with the other end, a source of pressurized air and a source of helium. The method comprises guiding the sampling probe into the inner pipe to its distal end, purging the inner pipe with pressurized air, filling the annulus defined between the inner and outer pipe with helium, and then detecting the presence of helium within the inner pipe with the probe as it is pulled back through the inner pipe. The length of the tube at the point where a leak is detected determines the location of the leak in the pipe.
U.S. Pat. No. 5,301,538 to Recla (Teledyne Industries) discloses a method and apparatus in which dual sensor detector tube systems are installed along the path length of a pipeline, storage tank, or other fuel system to be monitored for leaks. One sensor detector tube system is constantly evacuated and passed through a sensor detector to test for presence of vapors or gaseous indications of large leaks. Concurrently, the other sensor detector tube system is permitted to lie dormant for a predetermined period to absorb and receive vapors or gases from medium-to-small leaks that are too small to be detected by the continuous air flow system of the first tube. Periodically the flow of the two-sensor detector tube system is interchanged to evacuate the dormant tube which then becomes the continuously aspirated system. Signal output from the sensor detectors provide data to the control alarm system which compares concentration profiles of current on-going tests with recorded profiles of prior tests, and any significant deviation is recognized as a problem, sounding an alarm. The system provides direct data for location of small leaks, with significant indications for location of large leaks.
U.S. Pat. No. 5,072,622 to Roach et al discloses a pipe system comprising at least one lined pipe section. This lined pipe section comprises an outer pipe having an exterior wall, a thermoplastic liner having an exterior surface flush and in tight engagement with the interior wall of the outer pipe, and at least one groove located in the exterior surface. The groove and the interior wall of the outer pipe define at least one passageway at the interface of the interior wall of the outer pipe and the exterior surface of the liner with which a leak detector may be associated. This allows detection of leakage due to holes or perforations created by corrosion or other means in the outer pipe or due to cracks or holes in the inner liner. This invention therefore provides an effective dual containment system since the integrity of both the inner liner and the outer pipe may be monitored.
U.S. Pat. No. 4,450,711 to Claude (Technigaz, France) discloses a pipeline for conveying a fluid submerged in an ambient fluid medium which has an inner conveying tube surrounded by an outer tube radially spaced from the inner tube and defining a continuous annular space therebetween. A leak conductor communicating with a leak collector and surrounding the inner conveying tube conducts leaks of conveyed fluid to the leak collector. The leak collector has a reduced cross-section and is divided into adjacent sections isolated in a fluid-tight manner relative to each other and extending throughout the length of the tube. Each of the sections has at least a single valve automatically opened by predetermined pressure of the conveyed fluid leaks for providing communication with the annular space surrounding the inner tube. A leak detecting system in the annular space includes a sweeping device for sweeping the annular space with an auxiliary fluid and an analyzing device for analyzing the composition of the auxiliary fluid at one accessible end of the pipeline and at spaced locations along the pipeline. A warning device connected to the analyzing device is operable when the analyzing device indicates a predetermined pressure.
Each of the above-mentioned patents teaches methods and devices for determining the presence of leaks in lined piping and equipment or double walled piping. Since liners in process equipment are often used to protect the structural shell from highly corrosive media, loss of liner integrity may cause sudden leakage which is accompanied by considerable risk of health, safety and environmental consequences. As such, leak detection has little value for lined equipment in highly corrosive services. Therefore, a safe, cost-effective method is needed to identify suspect and/or defective liners substantially before leakage is imminent.
The present invention teaches that the semi-permeable membrane properties of the polymeric liner provides a rational basis for its inspection. The natural permeation behavior of volatile gasses is monitored and the measured quantity is compared against expected values for virgin materials. Any change of liner permeability may be related to changes in its physical condition. Any observed changes in membrane properties of the liner provides a basis for deducing certain aspects of its integrity.